During various well-working operations, it is frequently necessary to form a hard set plug in the well bore. When the well bore is either full or partially full of a liquid, it is difficult to form the plug at the desired location within the well bore.
As noted in U.S. Pat. No. 3,447,608 to Fry et al., in setting a cement plug within an open hole which contains mud, the more dense plug material may fall through the mud into the lower part of the bore hole. In attempts to solve this problem by lowering the density of the cement slurry used as the plug material, the plug which was formed lacked sufficient compressive strength due to the low-density slurry used in forming the plug.
In attempting to deal with the problem of placing a plug within a well bore which contains liquid, the Frey et al., patent utilizes a temporary plug which is made of low-density material. The low-density temporary plug has a density slightly greater than the mud and is suspended in the drilling mud within the bore hole. After setting of the temporary plug material, cement is then pumped into the bore hole in contact with the temporary plug which supports the cement as it hardens to form a permanent plug.
The problem of forming a plug within a well bore which contains liquid is also dealt with in U.S. Pat. No. 3,844,351 to Sutton et al. In the procedure of Sutton et al., the plug material is formed of a lightweight cement which preferably has a density of not more than about 80 pounds per cubic foot. The plug material used by Sutton et al., contains a foam stabilizer, in addition to water and dry cement, such that air may be entrained within the cement composition in the desired amount to lower the density of the cement composition. The aerated cement composition is then placed on the surface of the liquid within the well bore with the cement composition being buoyed up by the liquid with air at atmospheric pressure above the surface of the cement composition.
In the prior art procedures exemplified by the Fry et al., and Sutton et al., patents, the plug material is positioned at the surface of a liquid within the well bore. However, it is frequently desirable to place a plug at a location which is below the surface level of a liquid within a well bore. For example, due to the porosity of the formation and liquid pressure within the formation, a liquid may rise within a well bore to a given level. Under these circumstances, the prior art procedures of the Fry et al., and the Sutton et al., patents would be of no avail in forming the plug at the desired location since the desired location is below the surface level of the liquid within the well bore. The prior art of Fry et al., patent does not form a gravity stratification of a plug between a lower, more dense phase and an upper, less dense phase that compresses the plug.
There is a need for a procedure in which a plug may be formed within a well bore at a level that is below the surface level of liquid within the well bore. Additionally, as referred to in the Fry et al., patent, there is a need for a procedure for forming a plug within a well bore such that the plug has greater strength and forms a better bond with the surface of the well bore.